Voltage controlled oscillators (VCOs) are well known and widely used in the electronics industry. Within the digital communications field, VCOs are used in a variety of applications. Such applications include, for example, frequency synthesizers, clock generation and the like. VCOs are typically designed to perform within a given set of boundary conditions and to perform according to a specified standard. Typical conditions include, for example, performance over operating temperature ranges, sensitivity to noise, output sensitivity to interference, and the like. Typical performance standards include, for example, output signal frequency stability, output signal programmability and the like.
A typical prior art VCO circuit generates an oscillating output signal having a specified frequency. The signal can have several different wave forms (e.g., square, saw tooth, triangular, etc.). The frequency of the output is tunable and is a function of an input voltage, an external resistance or capacitance, or the like. The type of application in which the VCO circuit is used dictates its operating conditions and performance requirements.
In addition, the type of application also largely determines type of fabrication technology used to manufacture the VCO. A large number of modern digital integrated circuits are fabricated using well known and widely used CMOS technology. Where the VCO circuit is included in a CMOS IC (integrated circuit), it is usually fabricated in CMOS (e.g., fabricated using CMOS process technology).
There is a problem, however, when the application in which the overall IC is used requires the VCO circuit to operate at very high speed with low power while maintaining low noise and low PSS (Power Supply Sensitivity). For example, where the IC is part of a high speed serial transmission system (e.g., high speed wireless transmission systems) it is important that the output frequency of the VCO circuit be stable at high frequency, and be a consistent function of the control inputs while the output frequency exhibits very low phase noise.
Consequently, for these very high performance applications it is important that the VCO circuit provide a very stable, low jitter output signal at high frequencies with low power. However, prior art CMOS VCOs cannot reliably function at such high frequencies with low power while maintaining low jitter and low PSS.
Thus, what is required is a CMOS VCO circuit which solves the high speed operation problems of the prior art. What is required is a circuit capable of reliable operation at high frequencies with low power consumption while exhibiting very low phase noise on the output signal. The present invention provides an advantageous solution to the above requirements.